cmd/link: -race doesn't work by default on FreeBSD with cgo

[prattmic]

On our freebsd-amd64 builders (14.2-RELEASE-p2):

package main

/*
int foo(void) { return 42; }
*/
import "C"
import "fmt"

func main() {
        fmt.Println(C.foo())
}

$ go build cgo.go
$ ./cgo
42
$ go build -race cgo.go
$ ./cgo
This sanitizer is not compatible with enabled ASLR and binaries compiled with PIE
$ go build -race -ldflags=-linkmode=internal cgo.go
$ ./cgo
42

i.e., using cgo defaults to external linking, but it seems that external linking breaks -race mode.

cc @golang/freebsd

[gopherbot]

Change https://go.dev/cl/674160 mentions this issue: runtime: skip testprogcgo tests in race mode on freebsd

[gabyhelp]

Related Issues

• cmd/go, cmd/link, runtime: enable PIE build mode, cgo race tests on FreeBSD #24546 (closed)
• runtime: go test -race on FreeBSD with ASLR reports ok without running any test #65425

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[cherrymui]

In internal linking mode with disable ASLR under -race, https://cs.opensource.google/go/go/+/master:src/cmd/link/internal/ld/elf.go;l=800
We probably should pass a flag to the external linker to do so. Reading FreeBSD's ld manpage, its not clear to me what flag to pass. @golang/freebsd do you have any idea?

Thanks.

[paulzhol]

The equivalent would be calling elfctl -e +noaslr <file>.
I'm not sure LLVM ld.lld has anything explicit for setting a NT_FREEBSD_FEATURE_CTL note (maybe a custom linker script with a .note section).
@igalic @emaste Maybe you could help?

[emaste]

There is indeed no built-in support in ld.lld for configuring NT_FREEBSD_FEATURE_CTL. The only supported approach is having elfctl modify the linked binary.

The bits in the note are documented in elf(5):

     NT_FREEBSD_FEATURE_CTL (Value: 4)
           Contains a bitmask of mitigations and features to enable:

           NT_FREEBSD_FCTL_ASLR_DISABLE (Value: 0x01)
                 Request that address randomization (ASLR) not be performed.
                 See security(7).

           NT_FREEBSD_FCTL_PROTMAX_DISABLE (Value: 0x02)
                 Request that mmap(2) calls not set PROT_MAX to the initial
                 value of the prot argument.

           NT_FREEBSD_FCTL_STKGAP_DISABLE (Value: 0x04)
                 Disable stack gap.

           NT_FREEBSD_FCTL_WXNEEDED (Value: 0x08)
                 Indicate that the binary requires mappings that are
                 simultaneously writeable and executable.

It would be fairly straightforward for us to propose a patch adding support to ld.lld but it hasn't been done yet.